Semiconductor devices such as DRAMS and EPROMS are typically formed on semiconductor wafers, e.g., silicon wafers. During the fabrication of such devices, the wafers are handled and positioned many times as they are moved, e.g., from a carder to a position in a cleaning boat, to an oxidation boat, to a tester, etc. During fabrication processing, the wafer must be handled and supported with great care to minimize wafer damage. Wafers may be supported during processing using an arrangement as shown in FIG. 1.
With reference to FIG. 1, the wafers 101 may be supported by three supports 102 (two shown) connected to a support boat 103. As the wafers are processed, the high temperature of some processing steps, such as oxidation, diffusion, annealing and low pressure chemical vapor deposition (LPCVD), lowers the shearing yield stress of the wafers. In this context, the shearing yield stress of the wafer is the shearing stress at which the wafer shears. For example, at 1100.degree. C., a silicon wafer has a shearing yield stress of approximately 0.07 Kgf/mm.sup.2. If the shear stress between one of the supports 102 and the semiconductor wafer exceeds this shearing yield stress, crystal defects such as slips are generated, adversely impacting on device performance.
Using the support arrangement shown in FIG. 1, for wafers having a relatively small diameter, for example, a diameter of 200 mm, the shear stress between each of the supports and the wafer caused by the wafer weight itself is approximately 0.028 Kgf/mm.sup.2 when the supports are positioned approximately 12.5 mm (distance shown as "x" in FIG. 1A) from the edge of the wafer. As a result, there is little danger that the shear stress between the supports and the wafer will exceed the yield point during high-temperature processing and cause crystal defects in the wafer.
However, larger wafers experience a larger shear stress as the result of heavy weight of the wafer itself. For example, using the support arrangement shown in FIG. 1, the shear stress between each of the supports and a wafer having a 300 mm diameter is approximately 0.062 Kgf/mm.sup.2 when the supports are positioned approximately 18.8 mm (distance shown as "x" in FIG. 1A) from the edge of the wafer, and the shear stress between each of the supports and a wafer having a 350 mm diameter is approximately 0.078 Kgf/mm.sup.2 when the supports are positioned approximately 21.9 mm (distance "x" in FIG. 1A) from the edge of the wafer. Therefore, there is a greater danger that the shear stress on the wafer (caused by the weight of the wafer) during high temperature processing will exceed the shearing yield point.
One method of reducing the shear stress on the wafer is to increase the number of supports on which the wafer is positioned. However, it is very difficult to support a wafer equally using more than three supports. Specifically, the wafer may bend (warp) during high temperature processing and particularly during temperature ramp ups (when the temperature of the wafer is rapidly increased) and ramp downs (when the temperature of the wafer is rapidly decreased), resulting in uneven support of the wafer by the supports. For example, as illustrated in FIG. 1A, warping of a wafer 101 during processing may result in increased shearing stress on the wafer at point B because the wafer is no longer supported at point A. Since the wafer is not supported equally by all of the supports, shear stress may reach critical levels at the supports that support larger percentages of the wafer's weight. It is preferable to evenly distribute the weight of the wafer, for example, at points C and D shown in FIG. 1B. However, known support methods do not provide for even support of a wafer as the wafer warps during processing.